1. Field of the Invention
The present invention generally relates to a binder system for use in the field of forming ceramics or ceramic bodies and a method of producing ceramics or ceramic honeycomb bodies utilizing such a binder system. More particularly, the invention is concerned with a binder system that contains a branched paraffin-based organic liquid component and the use of that branched paraffin-containing binder system in a method for producing ceramic honeycomb bodies
2. Discussion of the Related Art
Binders and binder systems useful for the manufacture of products from powdered materials, e.g., from particulate ceramic materials, must meet a number of requirements. For example, the binder and binder systems must be compatible with the ceramic material such that a flowable dispersion comprising a relatively high loading of the ceramic material in the binder may be provided. In addition, the "green" preform produced by shaping the dispersion of ceramic powder in the binder should have reasonable strength such that it can be handled.
For desirable "burnout" or removal of the binder, the binder should be removable from the shaped ceramic part without incurring distortion or breakage of the part. Furthermore, the binder-free preform should have at least a minimum level of strength, yet be sufficiently free of binder residues that defect-free consolidation is readily achievable.
The formulation of binders meeting these requirements is complex and a large number of different binder formulations have been disclosed in the prior art. Recently, cellulose ether binders have been favored for use in the forming articles of various shapes; i.e., honeycomb substrates. The mixtures are intimately blended and homogeneous and result in the green body having good integrity in size and shape, as well as uniform physical properties. In addition to the binders, these powder mixtures typically include certain organic additives, including for instance, surfactants, lubricants, and dispersants that function as processing aids to enhance wetting thereby producing a uniform batch.
Recently, there has been an increase in the demand for thinner walled, higher cell density cellular structures, complex shaped product, and products having a large frontal area. Thin walled and complex shaped products produced, utilizing the current binder technology, i.e., cellulose ether binders, are extremely difficult to handle without causing shape distortion because of the low strength of the "green" preform.
One proposed solution disclosed in co-assigned U.S. Pat. App., Ser. No. 60/069,637 (Chalasani et al.) involves the use of a powder mixture, for forming honeycomb structures, that includes powder materials, binder, solvent for the binder, surfactant, and a non-solvent (with respect to the binder, solvent and powder materials), preferably a hydrophobic non-solvent. This powder mixture is mixed, plasticized and shaped to form a green ceramic preform body having improved wet green strength and thus is especially suitable for use in the processing of thin walled honeycomb structures.
While this Chalasani reference provides significant advances in the capability of the art to form complex, thin-walled ceramic honeycomb bodies through extrusion, including increased extrudate stiffness for equivalent extrusion pressures over those ceramic batches extruded without oil and an increased extrusion rate (2-3X), the inclusion of this non-solvent in the powder, e.g., light mineral oil, results in additional complications in the "burnout" or removal of the binder. Specifically, it is difficult to remove the binder components from the shaped ceramic part without incurring distortion or breakage of the part. Specifically, the strength of the material in the binder removal region (150 to 500.degree. C.) is considerably weaker compared to the standard non-oil batches. Furthermore, the corresponding dimensional changes in this region are also significantly larger due to the complex process of oil and binder removal. As a result, cracking becomes an important issue, especially when volume of the parts is large. As such, special considerations in the firing of the ceramic honeycomb must be undertaken to avoid cracking of the ceramic body; e.g., specially designed kilns, apparatus for volatile removal, reduced oxygen containing atmospheres and increased, complicated firing cycles.
An improved binder system for use in the formation of ceramic or other powder-formed greenware that overcomes the aforementioned firing problems is disclosed in assigned U.S. Pat. App. Ser. No. 09/321,013 (Beall et al.). The binder system disclosed therein comprises a binder, a solvent for the binder, a surfactant, and a component that is non-solvent with respect to the binder and solvent. The non-solvent component exhibits a lower viscosity than the solvent when containing the binder and comprises at least a portion of an organic liquid having a 90% recovered distillation temperature of no greater than about 225.degree. C. and more preferably less than 220.degree. C.; e.g., odorless mineral spirits. The benefit of using these organic liquids, including odorless mineral spirits, is that allows for its removal during the drying portion of the heating through evaporation. As such, at least a portion of the binder system is removed prior to the firing portion of the heating without any exothermic intensity associated with previous binder systems.
Recently it has been discovered that certain straight chain hydrocarbons and those branched chain hydrocarbon having predominately carbon chain distributions of less than 12 or and a 90% recovered distillation below about 205.degree. C.; (e.g. certain odorless mineral spirits) resulted in uneven flow fronts during extrusion in a twin screw mixer, and ultimately, resulted in regions where the extrudate exhibited buckled webs. As these regions typically result in reduced strength in the fired product and increased backpressure when the fired product is used in standard automotive exhaust applications, they are therefore undesirable. It was discovered that the use of these materials reduced the torque experienced by the extruder during mixing. This is thought to reduce the degree of mixing which results in inhomogeneity in the batch. These areas of inhomogeneity are thought to contribute to the uneven flow.
In light of the foregoing inconveniences experienced in the art, there remains a need to develop a binder system which permits a ceramic body to be formed and fired into a desired ceramic article without the presence of buckled webs and subsequent increase in back pressure upon use of the ceramic article in automotive exhaust.